Heretofore, the air/fuel ratio control apparatus of this type has been extensively known. In general, such an apparatus has an upstream-side air/fuel ratio sensor and a downstream-side air/fuel ratio sensor respectively interposed in exhaust passage parts on an upstream side and a downstream side with respect to a catalyst disposed in the exhaust passage of an internal combustion engine, and it calculates a sub-feedback correction magnitude on the basis of the difference between the output value of the downstream-side air/fuel ratio sensor and a predetermined downstream-side target value (by, for example, subjecting the difference to a proportional, integral and differential process (PID process)), while it calculates a main feedback correction magnitude on the basis of the difference between the output value of the upstream-side air/fuel ratio sensor and a predetermined upstream-side target value (by, for example, subjecting the difference to a proportional and integral process (PI process)). In addition, such an apparatus computes a command final fuel injection quantity which is obtained in such a way that a fuel quantity for obtaining a target air/fuel ratio (a command basic fuel injection quantity) as is acquired from an in-cylinder intake air quantity estimated on the basis of the running state of the engine (for example, an accelerator opening degree, an engine revolution speed, etc.) is corrected on the basis of the main feedback correction magnitude and the sub-feedback correction magnitude, and it instructs an injector to inject fuel of the command final fuel injection quantity, thereby to feedback-control the air/fuel ratio of a mixture which is fed into the engine.
Meanwhile, the catalyst (ternary catalyst) ordinarily has a so-called “oxygen occlusion function” owing to which, when the air/fuel ratio of exhaust gas flowing into the catalyst is a lean air/fuel ratio, the catalyst reduces nitrogen oxides (NOx) in the exhaust gas and keeps therein oxygen deprived of the nitrogen oxides, and when the air/fuel ratio of the exhaust gas flowing into the catalyst is a rich air/fuel ratio, the catalyst oxidizes HC, CO and the like unburned components in the exhaust gas by the kept oxygen. Accordingly, high frequency components of comparatively high frequencies in the fluctuations of the air/fuel ratio of the exhaust gas in the upstream of the catalyst, and low frequency components of comparatively low frequencies and comparatively small amplitudes (deviation magnitudes from a theoretical air/fuel ratio) can be completely absorbed by the oxygen occlusion function which the catalyst has, so that they do not appear as the fluctuations of the air/fuel ratio of the exhaust gas in the downstream of the catalyst.
On the other hand, low frequency components of comparatively low frequencies and comparatively large amplitudes in the fluctuations of the air/fuel ratio of the exhaust gas in the upstream of the catalyst are not completely absorbed by the oxygen occlusion function of the catalyst, and they somewhat later appear as the fluctuations of the air/fuel ratio of the exhaust gas in the downstream of the catalyst. As a result, there occurs a case where the output value of the upstream-side air/fuel ratio sensor and the output value of the downstream-side air/fuel ratio sensor become values which indicate air/fuel ratios deviating in directions opposite to each other relative to the theoretical air/fuel ratio. In this case, the air/fuel ratio control of the engine as based on a main feedback control (the main feedback correction magnitude) and the air/fuel ratio control of the engine as based on a sub-feedback control (the sub-feedback correction magnitude) interfere with each other, and hence, the favorable air/fuel ratio control of the engine cannot be performed.
For the above reason, the occurrence of the interference of the air/fuel ratio controls of the engine can be avoided, when the output value of the upstream-side air/fuel ratio sensor after the extent of frequency components that can appear as the fluctuations of the air/fuel ratio in the downstream of the catalyst have been cut from within the frequency components in the fluctuations of the output value of the upstream-side air/fuel ratio sensor (that is, low frequency components below a predetermined frequency) is used for the main feedback control.
On the basis of such knowledge, an engine control apparatus (air/fuel ratio control apparatus) stated in, for example, JP-A-5-187297 executes an air/fuel ratio control on the basis of a value obtained after the output value of an upstream-side air/fuel ratio sensor has been subjected to high-pass filtering, and the output value of a downstream-side air/fuel ratio sensor (in this example, a value obtained after the output value has been subjected to low-pass filtering). According to this, the occurrence of the interference of the air/fuel ratio controls of an engine as stated above can be avoided, and an air/fuel ratio control for the extent of fluctuations of an air/fuel ratio at or below a predetermined frequency that can appear as the fluctuations of an air/fuel ratio in the downstream of a catalyst (a substantial air/fuel ratio control) can be reliably performed by a sub-feedback control. Besides, high frequency components of or above the predetermined frequency in the fluctuations of the output value of the upstream-side air/fuel ratio sensor pass through a high-pass filter and therefore appear as the value obtained after the high-pass filtering. Accordingly, in such a case where the internal combustion engine is in a transient running state and where the air/fuel ratio of exhaust gas fluctuates greatly at the high frequencies of or above the predetermined frequency, an air/fuel ratio control for the fluctuations of the air/fuel ratio at or above the predetermined frequency (that is, a compensation for the abrupt change of the air/fuel ratio in the transient running state) can be performed rapidly and reliably by a main feedback control.
Meanwhile, in general, the difference between an in-cylinder intake air quantity, which is estimated in order to acquire a command basic fuel injection quantity, and an actual in-cylinder intake air quantity, and the difference between a command fuel injection quantity for an injector for injecting fuel and an actual fuel injection quantity (hereinbelow, these shall be generally termed the “error of the basic fuel injection quantity”) occur inevitably. In order that the air/fuel ratio of a mixture to be fed into the engine may be converged to a target air/fuel ratio while such an error of the basic fuel injection quantity is being compensated (concretely, that the steady-state deviation between the output value of the air/fuel ratio sensor and the predetermined target value may be made “0”), a process for calculating a feedback correction magnitude (that is, an integral process (I process)) needs to be executed on the basis of the time integral value of the deviation between the output value of the air/fuel ratio sensor and the predetermined target value, in at least one of the main feedback control and the sub-feedback control.
However, the high-pass filtering is a process for achieving a function equivalent to a differential process (D process). In the apparatus stated in the above document, accordingly, although the main feedback control executes a process containing the integral process (for example, a proportional and integral process (PI process)), the integral process cannot, in effect, be executed in the main feedback control. In this case, accordingly, the integral process needs to be executed in the sub-feedback control.
However, the change of the air/fuel ratio of the mixture to be fed into the engine appears somewhat later as the change of the air/fuel ratio of the exhaust gas in the downstream of a catalyst, under the influence of the oxygen occlusion function of the catalyst as stated before. Consequently, in a case where the error of the basic fuel injection quantity increases rapidly, this error of the basic fuel injection quantity cannot be immediately compensated by only the sub-feedback control, resulting in the problem that there occurs a case where an emission exhaust quantity increases temporarily.